The present invention relates to instrumentation for measuring high-intensity neutron and gamma radiation at elevated temperatures, and more particularly to a miniature ionization chamber predominantly used for monitoring inside the cores of nuclear reactors.
Detectors used for monitoring, controlling and protecting nuclear reactors provided with relatively small cores and moderate output are normally arranged outside of the core or inside the reactor vessel in specially cooled channels.
The development of high-energy power and research reactors operating high neutron fluxes (about 5.10.sup.14 neutrons/sq cm/sec) and at elevated temperatures (up to 700.degree.-800.degree.C) has made it urgent to monitor the intensity and distribution of the energy liberated inside the core, as detectors arranged outside the core are becoming less sensitive to local surges of the neutron flux. At the same time, the power of reactors has to be limited so as to protect fuel elements against uncontrolled surges of energy and prevent accidents.
This consideration has prompted intensive studies with a view to providing neutron detectors permitting monitoring inside the cores of nuclear power reactors.
The high cost and complexity of cooled monitoring devices have become another factor contributing to the development of high-temperature neturon detectors.
Meters based on the application of activation methods (activation of foil, wire, etc) are unsuitable for routine monitoring of energy liberation, especially in control and protection systems.
Among the great variety of the currently used neutron detectors for monitoring inside reactors, the widest application is found by ionization chambers and emission detectors.
Emission detectors are simple in design and reliable in operation. However, they are generally characterized by slow response, and those characterized by fast response produce a signal which becomes commensurable with background currents at temperatures of about 400.degree.-500.degree.C. This restricts their application in reactor control and protection systems.
Ionization chambers, on the other hand, are characterized by fast response and can operate at high temperatures. The neutron-sensitive coating of ionization chambers may comprise a combination of fissile isotopes, whereby an ionization chamber may operate in high thermal fluxes over a long period of time without its sensitivity being impaired.
All ionization chambers, regardless of their type, purpose and structure, are provided with a collecting electrode energized by an external power supply. This electrode is insulated from the other structural elements of the chamber. A decrease in the interelectrode insulation resistance to a certain value adversely affects the operation of the ionization chamber and may ultimately result in its failure.
In developing ionization chambers capable of operating at elevated temperatures and in high ionizing radiation fields, designers followed two directions: first, they tried to enhance the thermal radiation stability of the interelectrode insulation, second, they sought a solution of the problem of reducing leakage currents. In the United States, in France, and in the Federal Republic of Germany, for example, ionization chambers have been developed using insulating materials with a high dielectric constant and capable of operating at a temperature as high as 600.degree.C.
The use of insulators with a great surface area to reduce surface leakage currents or increase the volume of the insulating material used in the chamber also contributed to a higher thermal and radiation stability.
However, for ionization chambers currently used in nuclear engineering, the working temperature of 600.degree.C is practically the maximum above which they are ineffective. This is due to the fact that under the effect of intensive ionizing radiation fields and elevated temperatures the interelectrode insulation loses its insulating properties and becomes a source of background current limiting the temperature range of operation and reducing the reliability of the chamber. Even insulating materials featuring the highest thermal radiation stability known at present, based on superpure alumina, become electrically conducting at temperatures above 600.degree.C.
The search for new structural designs has lead to the development of a high-temperature ionization chamber with a guard electrode. However, the construction of this chamber solves the problem but partially, since the guard electrode made up of two rings interconnected through jumpers and arranged on spacers made of an insulating material is disposed internally of the chamber. In this embodiment, leakage currents are only eliminated inside the chamber and not at its input. As a result, it is impossible to reduce the size of the ionization chamber radially which substantially narrows the field of its application.
In some ionization chambers, the guard electrode is disposed at the input to the chamber, which in no way solves the problem of eliminating leakage currents inside the chamber. This does not permit extensively using such an ionization chamber, especially in reactors wherein the in-core temperature exceeds 600.degree.C.
Another small-size ionization chamber is known whose lead-in is made as a triaxial cable and whose gas-filled housing accommodates a collectng electrode electrically connected to the centre conductor of this cable, the electrode being fixed lengthwise with respect to the housing by means of spacers, and a guard electrode for reducing leakage currents appearing under the effect of ionizing radiation and high temperature.
The intermediate coaxial conductor of the triaxial cable being used in this ionization chamber as the guard electrode substantially (by 2 to 3 orders of magnitude) alleviates the requirements imposed on the insulation of this cable. However, the presence inside the chamber housing of a spacer made from an insulating material, and the absence of the guard electrode therein, restricts the field of its application to a temperature of about 600.degree.C and the disintegration of the insulating material under the effect of the ionizing radiation field shortens the life of the ionization chamber.